A significant patient concern in mammography is the discomfort the patient feels when the breast is compressed, typically between two rigid plastic surfaces, with sufficient force to spread out the breast tissues. The reasons for using such high compression include: (1) to make the breast thinner in the direction of x-ray flux and thereby reduce patient radiation exposure from the level required to image the thicker parts of an breast that is not compressed; (2) to make the breast more uniform in thickness in the direction of x-ray flux and thereby facilitate more uniform exposure at the image plane over the entire breast image; (3) to immobilize the breast during the x-ray exposure and thereby reduce image blurring; and (4) to bring breast tissues out from the chest wall into the exposure area and thus image more tissue.
Reason 2 has been relevant mainly to analog screen-film mammography imaging, which has a limited dynamic range and requires fairly specific radiation levels on the image receptor to avoid either under- or over-saturating the film. With the advent of digital mammography and its wide dynamic range, this requirement is no longer as compelling, because image processing can flatten the image post-exposure and provide a more uniform gray scale appearance even if the breast has significant thickness variations across the field of view. Breast tomosynthesis (tomo), an emerging modality that uses three-dimensional imaging, has an even lesser need for uniform breast thickness. There is therefore an opportunity to provide breast immobilization methods that are especially suited for digital mammography and tomosynthesis. Reason 1 is also less important for digital mammography and tomo, because these modalities typically can use higher average x-ray energies and keep radiation exposures low even for thicker breasts.
Standard compression methods for mammography use a movable, rigid clear plastic compression paddle. The breast is placed on a bottom breast platform that is flat, and the paddle is then compressed onto the breast, usually while the technologist is holding the breast in place and perhaps helps positioning it between the compression paddle and breast platform to ensure proper tissue coverage in the image receptor's field of view. One reason for the discomfort felt using this method is that the compression force is non-uniformly distributed throughout the breast. It is concentrated at the thickest portion of the breast, usually near the chest wall. The anterior portion of the breast, such as near the nipple, may not receive any compression force because the paddle may not even contact this portion of the breast.
Some compression paddles tilt, such as the paddle available in various sizes from Lorad of Danbury, Conn., a division of Hologic, Inc. of Bedford, Mass., under the trade name F.A.S.T. This tilting paddle provides more uniform compression across the breast, and more comfortable breast examinations. An example of such a paddle is illustrated in FIG. 11, as mounted in a mammography system commercially available from Lorad. It is illustrated in FIG. 12 as used to compress a patient's breast in a similar mammography system.
Other methods have been proposed and may have been used clinically to improve patient comfort. One is the use of relatively thin foam pads that are placed above and/or below the breast. The pad compresses during the compression procedure and may provide improved comfort by spreading out the pressure to a greater extent than using a hard-surfaced paddle and/or breast platform alone. One such pad system is proposed in U.S. Pat. Nos. 6,968,033, 6,765,984, and 6,577,702 and published U.S. patent application US 2003/0007597 A1, all assigned on their face to Biolucent, and is believed to be on sale by Biolucent. An example thereof is illustrated in FIG. 13 and FIG. 14, when placed on the breast platform. Another pad system is proposed in U.S. Pat. Nos. 6,850,590 and 6,975,701 and published U.S. patent applications US 2006/0050844 A1, US 2004.0156472 A1 and US 2003/0099325 A1, all naming as the inventor Benjamin M. Galkin. One disadvantage of pad systems of this type is that the pads are not transparent to visible light. This means that there is impaired visibility of the breast, if the pad is used above the breast. Visibility is important for the technologist to aid in breast positioning. Another disadvantage of this solution is that the pad needs to be made of fairly dense thin form, so as to provide at least meaningful deformability under the high pressures. It is believed that this foam can creates minor artifacts in the image, especially if the pad slips during positioning and does not cover the entire breast. Another limitation is that this pad is relatively expensive to manufacture, and this makes it more expensive to use, and that it may require excessive time to secure to and then remove from the breast platform and/or compression paddle. Yet another system for improving patient comfort has been proposed for a different purpose—to immobilize the breast during biopsy—by Scientific Biopsy (www.sbiopsy.com). It is illustrated in FIG. 15 and is understood to use a soft, trough-shaped support to cradle the breast and a flexible band that wraps on top of the breast to impose a holding force. A thin plastic sheet compressing a breast for ultrasound examination rather than for x-ray imaging is proposed in published patent application US 2003/0007598 A1 (see, e.g., FIG. 7 and paragraph [0115]) but no teaching could be found that the material is transparent to visible light or that the arrangement is useful for x-ray imaging or with a flat breast platform.
The known existing methods of improving patient comfort during breast mammography are believed to have been designed with analog screen-film mammography in mind, especially the requirements of such mammography for very thin, very uniform thickness in a compressed breast. However, newer modalities such as digital mammography and tomosynthesis have different needs and constraints, offer new challenges and new opportunities, and have allowed for reconsideration the old approaches. For example, tomosynthesis methods do not need to have the patient's breast compressed to a degree that would make it substantially flat because tomosynthesis is a 3d imaging modality. In digital mammography, the breast does not need to be made as flat as for analog mammography, because image processing can effectively flatten the image. The methods proposed in the patents and applications of Biolucent identified above are understood to involve relatively thin pads designed to accomplish essentially the degree of compression and flattening of the breast that is used in analog mammography; those pads are understood to be very thin to prevent much deviation from a thin flat breast. In addition, no indication has been found in those proposals to make the pads transparent to visible light and thereby facilitate breast positioning in the compression process.
This patent specification is directed to new approaches to patient comfort that are particularly suitable for such newer modalities, and is believed to overcome a number of disadvantages of the known compression approaches.
One non-limiting example of such new approaches to patient comfort in mammography and/or tomosynthesis involves the use of a specially adapted device to control, distribute and re-direct breast compression forces. The device preferably is between the patient's breast and the compression paddle, but can be between the breast and the breast platform, or such device can be used on each side of the compressed breast. Preferably at least some of the device is transparent to visible light to facilitate positioning the breast in compression, and typically the entire device, or nearly the entire device, is made of a material transparent to visible light. In a simple example, the device can be an air-filled pillow or bag of plastic material that is transparent to visible light. Preferably, the air pressure is selected such that the bag will conform to the breast in compression in mammography or tomosynthesis such that the path of x-ray flux through air in the pillow would be short or even zero-length at the thickest part of the compressed breast and progressively longer toward the thinner parts of the compressed breast. For example, the air inside the device can be essentially at atmospheric pressure and the amount of air inside the device can be such that the walls of the device are not tight but are somewhat wrinkled. At least a portion of one or both sides of the device that contacts the breast and/or the chest wall of the patient can be made of, or covered with, a material that is sticky or otherwise has a sufficiently high coefficient of friction with the patient's skin and/or the compression surface that contacts the device, to thereby prevent or at least significantly reduce slippage during compression between the device and the patient's skin, and/or between the other side of the device and the compression surface, and to help push into the x-ray field of view and the x-ray image more of the tissue adjacent the patient's chest wall. For example, a portion of the surface that would be at or near the chest wall is made sticky. The device can be made available in different sizes and at different levels of internal air pressure. It can be stored before use with air inside, or it can be stored flat, without air or with less air inside, and pumped with air to the desired pressure when it comes time to use it. The device can be a single-use device that can be used only for a single patient, or even for a single breast or a single view, and then discarded. Alternatively, the device can be a multiple-use device that is used for more than one x-ray view and/or more than one breast of the same patient, and/or can be used for several patients. A fresh length of a thin and transparent plastic film can be placed between the patient's breast and a multiple-use device if desired. A device can be temporarily secured to the compression paddle and or the breast platform with a suitable adhesive or by use of mechanical means such as hook-and-loop strips.
The device can comprise a single compartment filled with air (or another gas) or can have two or more compartments or chambers that can, but need not, differ in internal pressure and/or in internal volume. As a non-limiting example, the device can have one compartment filled with gas a higher pressure than another, or can have more than two compartments, each filled with has at respective pressure that can be the same as, or different from, that of other compartments that can have the same or different internal volumes. For example, a chamber or compartment near the chest wall of a patient can be filled to a higher pressure, or can have a greater internal volume, in order to compress the thickest part of the breast first and keep it from sliding from under the compression paddle as the compression force is raised. Alternatively, another construction of the device can be used to a similar end, for example, making the device tilted so it is thicker near the chest wall.
Another device that can be used to improve patient comfort in breast compression is a similar pillow or bag but filled at least partly with a liquid such as water rather that filled only with a gas such as air. In addition to the benefits of a gas-filled device discussed above, the device that is filled at least partly with a liquid such as water would provide some equalization of the x-ray path length that forms the breast image.
Another device for improving patient comfort in breast compression is a sheet of transparent and sufficiently strong material such as Mylar to take over some or all of the functions of a compression paddle. The material may be supported by rods or other holders that flank the breast and are supported and are movable in a manner similar to that of a compression paddle. Alternatively, the material may be supported between rods or other holders that are secured to the breast platform or another part of the mammography system, in which case the need for a compression paddle may be eliminated altogether. The material may be stretchable or otherwise deformable in ways that are not uniform across the portion thereof used to compress the breast. For example, one or more portions, or all the material, closer to the chest wall may stretch or deform less that portions or all the material further from the chest wall of the patient, to thereby compress the thicker part of the breast more. In addition, at least some portions of the material facing the breast may be made sticky, for example by making the material itself sticky or by coating it with a sticky substance, in order to eliminate or reduce slippage between the breast and the material.
Yet another device that improves comfort in breast compression is made wholly or in part of a compressible foam or similar material. The device may have substantially constant thickness in the direction of x-ray flux, or it may be shaped to provide a shallow depression that cradles the breast.
All of the devices listed above can be further improved by being provided with one or more marking portions that are more opaque to x-rays than the rest of the device and thereby affect the x-ray image in a beneficial way. The image artifact caused by a marking portion can be used for any one of a number of purposes. For example, it can serve as an indicator in the image that a device has been used to improve comfort during compression. It can further indicate the type of device that was used, its position and/or orientation relative to the breast or to another structure, and/or as another aid in imaging or interpreting the image. This deliberate image artifact can have characteristics that make it an acceptable addition to the image, even if it is within the outline of the imaged breast. For example, it can be at a place in the image that does not affect diagnostic or screening use or some other use of the image, it can be removed by computer-processing the image (since the characteristics of the artifact are known), or the artifact may inherently be absent from processed images based on the raw x-ray data (for example, the artifact may be positioned such that it is not seen in reconstructed tomosynthesis slice images that are of medical or other interest. For example, the artifact may show up in an image as a straight line shadow that can be safely ignored in assessing the image, or as a shadow in some other shape that can be ignored. The artifact may appear as a shadow of a shape that contains useful information, such as a circle, a rectangle or a trapezoid, for example, that evidences positional, orientation, or some other information about the device, the breast, or the imaging procedure. The artifact-causing substance can be a part of the same substance of which the rest of the device is made but is made thicker or denser at desired location. Alternatively, a different substance can be used, such as a strip or some other shape of aluminum or another substance that is more x-ray opaque than the rest of the device and is incorporated in or attached to the device. As a non-limiting example, an adhesive-backed strip of aluminum can be attached to the device, at a desired place on the device, before compression or even at some stage of the compression or after compression and before taking an x-ray image.